I don't know of zoo-eating stars, but there are mini asterina stars that can go after sps. Do you see any little snails or bugs at the base of your zoos? The bast way to find out is at night when the polyps retract, take a flashlight over the colonies and see if anything is sucking them out from the base.
Could be PO4, NO4, NO3, NO2, Ph, salinity, etc...how are your params?